1. Field of the Invention
The present invention relates to vent dampers for heating apparatus such as fireplaces, wood stoves, water heaters and gas-fired furnaces. More particularly, the present invention relates to thermally activated vent dampers which automatically open and close in the presence or absence of heat.
2. Description of the Prior Art
It has been known in conventional heating apparatus designs to incorporate thermally activated vent dampers above the combustion chamber either within the flue or at a point where the flue and the combustion chamber intersect. During periods where there is no heat produced in the combustion chamber, such dampers will remain closed to prevent heat loss from the internal room environment due to convection currents directed up the flue. It is also well known in the art that for a thermally activated vent damper to operate efficiently, the thermally responsive element of the damper must have an adequate response time so that the vent damper moves to a closed position shortly after combustion ceases in order to prevent unnecessary heat loss. Similarly, the thermally responsive element should possess an equally prompt response time so that the vent damper moves to an open position within an adequate time frame after combustion has commenced in order to allow the gases resulting from combustion to properly access the flue.
Many of these thermally responsive elements are made from a bimetallic material consisting of two metals each having different coefficients of expansion. Upon exposure to a sufficient increase in temperature, these metals expand at differing rates causing the thermally responsive element to undergo some sort of flexion or shape alteration. It is known that the direction, degree and rate of alteration or flexion is a function of the original shape of the thermally responsive element, the gauge of the materials used and the type of materials as well as their respective quantities.
Typically, the bimetallic elements used in vent dampers are subject to damage due to overheating as well as the extreme changes in temperature in which they must operate repeatedly over a number of years. It is known that this damage often occurs in the form of metal fatigue due to stresses placed on the thermally responsive elements during extended high temperature exposure. In fact, the damper assemblies disclosed in U.S. Pat. No. 4,386,731 to Barth and U.S. Pat. No. 4,460,121 to Hedrick are directed to relieving some of the stresses which thermally responsive elements experience during operation.
Metal fatigue is an extremely dangerous problem in thermally activated vent dampers as it may ultimately lead to the breakage of the thermally activated element due to the existing weight of the damper member or forces exerted during normal flue cleaning or maintenance operations. To compound the problem, these vent dampers are concealed from view and are difficult to inspect. One of the major shortcomings of thermally activated vent dampers utilizing a bimetallic spring or other thermally activated biasing elements is their lack of a fail-safe configuration. Upon breakage of the thermally responsive element, the flue damper will remain in a closed position unless acted upon manually. Consequently, combustion gases will be prevented from properly accessing the flue portion of the heating apparatus, creating an extremely hazardous situation.
Some thermally activated vent dampers use a very simplistic design having a bimetallic flap as a damping means. Typically, one end of the flap is secured in position against the flue or a support member while the other end is free to undergo movement. At ambient temperatures, the flap remains closed usually resting on a stop member positioned somewhere in the flue. Upon exposure to elevated temperatures, the free end of the bimetallic flap undergoes flexion and moves into a position which allows the gases resulting from combustion to properly access the flue. Upon cessation of combustion, the bimetallic flap returns to a closed position. One of the drawbacks of this type of arrangement is a poor response time due to the amount of heat build up necessary to induce a response in the larger surface area and thickness of the flap relative to other types of thermally activated vent dampers which typically have a better response time because they employ bimetallic coils or springs to move a solid damper member.
Another well known problem in vent damper assemblies, generally, involves friction occurring at points where moving parts operatively contact other structures. One source of such friction is the creation and deposition of residual particulate matter such as creosote which occurs as a by-product of the combustion of certain carbon-containing fuels. These deposits, some of which are extremely tar-like in consistency, tend to accumulate on the vent damper assembly as well as the flue walls inhibiting operative movements. An additional source of friction stems from the deterioration of the metal components within the vent damper assembly. Despite the use of corrosive resistant metals such as stainless steel or aluminized metals, high temperature operating conditions in conjunction with the corrosive properties of the flue gas ultimately lead to microscopic as well as macroscopic surface deformations including pits, fissures and depressions which increase the coefficient of friction and consequently inhibit movement.
It is well known in the art that the expansive properties of the metals themselves can also contribute to friction. As previously mentioned, repeated exposure to extreme changes in temperature over the years can result in metal fatigue and deformation. When these phenomena occur, stress points can develop in the vent damper assembly at sites where moving parts contact other structures despite efforts in design calculations and measurements to accommodate these changes. These stresses become magnified as the metal components of the vent damper assembly undergo normal expansion upon exposure to the elevated temperatures associated with combustion. The result is a dramatic increase in the coefficient of friction which consequently inhibits movement.
Increased friction in a thermally activated vent damper assembly is detrimental as it interferes with adequate response times. It is common knowledge in the art that any vent damper assembly possessing numerous moving parts has a greater potential for frictional inhibition of proper operation merely due an increased number of sites where moving parts contact other structures. Most thermally activated vent damper assemblies to date have damper members which are suspended in some fashion from a support member. Typically, the support member either rotates within the flue thereby rotating the damper member or alternatively the damper member rotates around the support member utilizing some hinge-type mechanism. Either approach creates a potential site for increased friction due to residual particulate accumulations or structural deformations.
Additionally, many thermally activated vent dampers are of a construction that prohibits deployment in a fireplace design where space is limited. For example, certain fireplace designs incorporate heat exchange mechanisms above the combustion chamber to provide for a more efficient delivery of heat to the surrounding internal room environment. Consequently, damper designs which require a wide range in movement of the damper member in order to function properly cannot be deployed in fireplace assemblies having limited space available for a damper mechanism.